Trona is a mineral that contains about 85-95% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). A vast deposit of mineral trona is found in southwestern Wyoming near Green River. This deposit includes beds of trona and mixed trona and halite (rock salt or NaCl) which covers approximately 2,600 km2. The major trona beds range in size from less than 428 km2 to at least 1,870 km2. By conservative estimates, these major trona beds contain about 75 billion metric tons of ore. The different beds overlap each other and are separated by layers of shale. The quality of the trona varies depending on its particular location in the stratum. Because trona is found contiguous to shale, the trona ore frequently contains shale impurities. Because shale impurities are undesirable in the product, there is a need to develop processing technologies to remove such impurities from trona.
U.S. Pat. No. 5,736,113 to Hazen et al. discloses a process for recovering a high-purity saline mineral from an ore containing the saline mineral, such as trona, and impurities. The process includes separating a first portion of impurities from the trona by an electrostatic separation method at a temperature between about 25° C. and about 45° C. The electrostatic separation methods are based on subjecting the ore to conditions such that materials of different electrical conductivities separate from each other. The process discloses an optional magnetic separation step before or after the electrostatic separation.
U.S. Pat. No. 5,911,959 to Wold et al. discloses a process for the purification of saline minerals having insoluble impurities. The process includes calcining trona to form sodium carbonate, sizing the feedstream into a large size fraction and a small size fraction, separating the large size fraction into a first recovered portion and a first impurity portion by a dry separation method, separating the first impurity portion into a second recovered portion and second impurity portion by a wet separation method, and separating the small size fraction into a third recovered portion and third impurity portion by a wet separation method. The dry separation method may be accomplished by density separation, magnetic separation, or electrostatic separation.
U.S. Pat. No. 6,092,665 to Schmidt et al. discloses a process for recovering a saline mineral from an ore containing the saline mineral and impurities. The process includes the steps of separating a first portion of impurities from the ore by density separation, electrostatically separating a second portion of impurities from the ore, and magnetically separating a third portion of impurities from the ore. In another aspect, the process includes the steps of calcining the ore and subsequently separating a first portion of impurities by density separation.
Equipment for separating magnetic particles from non-magnetic material is known in the art. In particular, Outokumpu Technology, Inc. of Colorado provides a magnetic roll separator. A brochure for the Improsys® Rare-Earth Roll Separator describes the use of a rare earth roll magnet to separate magnetic material from a non-magnetic material. The material to be separated is fed onto a separator belt that moves over the magnetic roll. Magnetic particles are attracted towards the magnetic roll, while nonmagnetic or diamagnetic particles flow unhindered in a trajectory away from the roll. The Outokumpu Technology brochure suggests that the magnetic roll separator can be used in many dry particle applications, including glass and ceramic raw materials (e.g., silica sands, etc.), heavy mineral sands, refractory raw materials, abrasives, fillers, potash salt, phosphate ores and many other materials, including metals, mineral ores and products. This brochure does not describe the use of trona. The brochure describes the optional use of an electrostatic system to prevent dust build-up.
While work has been performed in the beneficiation of trona, there is a need to improve the performance of magnetic separation processes to obtain efficient beneficiation of trona without the additional use of electrostatic precipitation and other separation techniques.